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We report the characterization of rice OsHKT1 (Oryza sativa ssp. indica) homologous to the wheat K+/Na+-symporter HKT1. Expression of OsHKT1 in the yeast strain CY162 defective in K+-uptake restored growth at mM and µM concentrations of K+ and mediated hypersensitivity to Na+. When expressed in Xenopus oocytes, rice OsHKT1 showed uptake characteristics of a Na+-transporter but mediated transport of other alkali cations as well. OsHKT1 expression was analysed in salt-tolerant rice Pokkali and salt-sensitive IR29 in response to external cation concentrations. OsHKT1 is expressed in roots and leaves. Exposure to Na+, Rb+, Li+, and Cs+ reduced OsHKT1 transcript amounts in both varieties and, in some cases, incompletely spliced transcripts were observed. By in situ hybridizations the expression of OsHKT1 was localized to the root epidermis and the vascular tissue inside the endodermis. In leaves, OsHKT1 showed strongest signals in cells surrounding the vasculature. The repression of OsHKT1 in the two rice varieties during salt stress was different in various cell types with main differences in the root vascular tissue. The data suggest control over HKT expression as a factor that may distinguish salt stress-sensitive and stress-tolerant lines. Differences in transcript expression in space and time in different lines of the same species appear to be a component of ion homeostasis correlated with salt sensitivity and tolerance.
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Based on their kinetic behaviour, the AKT/KAT family of inwardly rectifying K+ channels seems to represent a low-affinity component of the uptake machinery although this assignment has recently been questioned (Hirsch et al., 1998; Spalding et al., 1999). Expression of the root-specific K+ channel AKT1 was reported for both µm and mm external K+ concentrations (Lagarde et al., 1996). Also, growth of an Arabidopsis AKT1-mutant was reduced at µm K+ concentrations indicating that AKT-type K+ channels are not only involved in low- but also in high-affinity root K+ uptake (Spalding et al., 1999). The selectivity of these channels for K+ over other alkali metal ions is variable, but most are characterized by efficient Na+ exclusion and their contribution to sodium uptake seems to be minimal (Amtmann and Sanders, 1999; Blumwald et al., 2000; Maathuis et al., 1997).
A member of a presumably high-affinity K+-uptake system has been described in the wheat HKT1 protein (Rubio et al., 1995; Schachtman and Schroeder, 1994). It could also be a major route for the entry of Na+ into roots, according to its specificity as a K+/Na+ symporter (Rubio et al., 1995). Because many conclusions about HKT1 derive from functional analysis in yeast and Xenopus oocytes, HKT's action in cells of the root may be different, may be restricted to specific cells, or its activity may be masked or counteracted by the presence of other transport systems (Maathuis and Sanders, 1995, 1997; Maathuis et al., 1997; Tanner and Caspari, 1996). The presence of wheat HKT1 leads to Na+ uptake in high-Na+ media and, if expressed and active in roots, could significantly increase Na+ toxicity. Thus, a characterization of HKT-type transcripts and proteins in additional models is appropriate.
With the characterization of three K+-uptake systems, views about ion uptake are changing. Several protein subfamilies of the AKT and KUP families exist, each with different isoforms, and seemingly with gradual transitions in their kinetic and biochemical characteristics. In addition, ion specificity varies not only between the members of a family in one species, but also for orthologous family members among different species.
Our interest in K+ uptake systems is based on the goal of understanding the mechanisms that confer salinity tolerance to land plants. We have concentrated on the rice homologue of wheat HKT1 because of the large dataset available on rice salinity stress responses based on physiological experiments (Flowers and Yeo, 1995; Garcia et al., 1995; Yeo et al., 1990). Also, a number of varieties and breeding lines are available that are characterized with respect to K+ uptake and how salinity stress tolerance is acquired. We decided to utilize the Na+ uptake characteristics of two rice cultivars, the salt-tolerant line Pokkali and the salt-sensitive line IR29, for a comparative analysis of the function of rice HKT1. We have characterized a transcript, OsHKT1, encoding a plasma membrane protein with homology to the wheat high-affinity K+ transporter HKT1. As we will show, Pokkali and IR29 exhibit significant differences in their ability to take up or exclude Na+. Expression of OsHKT1 in both lines is affected by the presence of high concentrations of alkali ions, which repress the HKT transcript. However, repression was less pronounced in the root and leaf vascular tissues of the salt-sensitive IR29 compared to amounts present in the salt tolerant Pokkali. A similar conclusion has recently been drawn by Horie et al. (2001) in a comparison between lines Nipponbare and Pokkali. Taking their analyses and our results, that include different characterizations, it seems that HKT1 gene expression characteristics in the two varieties are significantly correlated with ion homeostasis.
Rice varieties with different K+-uptake characteristics
K+ and Na+ accumulation was analysed in two lines of indica rice (Oryza sativa): IR29 (salt-sensitive) and Pokkali (salt-tolerant), adapted to either 4 mm K+, 0.1 mm K+, or medium without K+ added (referred to as 0 mm K+ in the following) that contained 10–16 µm K+ from impurities in other salts (Figure 1). Under control conditions, leaf Na+ and K+ concentrations between the varieties were not significantly different when the plants were grown at 0, 0.1, and 4 mm of K+, respectively. Pokkali and IR29 showed, however, different K+ and Na+ uptake characteristics when the plants were salt-stressed at different external K+ concentrations. Both lines accumulated Na+ very efficiently in leaves but not roots when salt-stressed. When adapted to 4 mm and 0.1 mm external K+, IR29 plants accumulated more than 1000 µmol g−1 FW Na+ in leaves following 72 h of salt treatment whereas Pokkali plants excluded Na+ and had a leaf Na+ contents of about 30% compared to IR29. In the absence of K+ in the medium Pokkali accumulated less Na+ than IR29 during short-term treatment of 24 h NaCl. After 72 h of salt treatment, however, the leaf Na+ contents were similar in both rice lines with approximately 700 µmol g−1 FW. Na+ uptake was generally less in the salt tolerant rice line Pokkali, an effect that was dependent on the presence of external K+ in the nutrition medium.
A rice HKT1-homologue
Based on the nucleotide sequence of a partial rice cDNA clone (Cornell Collection RZ405) homologous to the wheat high-affinity K+ transport protein HKT1 (Accession No. U16709), we used RT-PCR amplification, cDNA library screening and 5′RACE extension to obtain a full-length transcript of the rice homologue. The cDNA sequence of OsHKT1 (AF313388) isolated from the rice line IR36is 1827 nucleotides in length. The 5′UTR of OsHKT1 is 60 nucleotides in length and contains a consensus sequence for eukaryotic translation start sites (Kozak, 1984). The 3′-end is 173 nucleotides in length and includes several putative polyadenylation sites. OsHKT1 includes an open reading frame encoding a protein of 530 amino acids for which a location in the plasma membrane is predicted (http://psort.ims.u-tokyo.ac.jp). The theoretical pI of OsHKT1 is 9.43 and the molecular weight is 59.3 kDa (http://www.expasy.ch). An analysis of the reading frame predicted a protein characterized by 12 hydrophobic domains, which fits the four-MPM model proposed by Durell et al. (1999). Kato et al. (2001) have convincingly shown this membrane topology for AtHKT1. A comparison with the wheat HKT1deduced protein sequence indicated co-linearity of the two proteins with high sequence identity in the hydrophobic regions. The deduced amino acid sequence of rice OsHKT1 is 65% identical to that of wheat HKT1(Schachtman and Schroeder, 1994), with an overall similarity of 77%. OsHKT1 and the Arabidopsis homologue AtHKT1 (Uozumi et al., 2000) show 39% sequence identity and 56% sequence homology. Recently, DNA sequences of OsHKT1 have been obtained from japonica rice Nipponbare and indica line Pokkali (Horie et al., 2001). The deduced amino acid sequences of both are identical to OsHKT1 isolated from line IR36reported here. Between Nipponbare and the two indica lines four nucleotides are different in the coding region and the 5′ non-coding region of Nipponbare shows a four-nucleotide deletion. All rice HKT sequenced so far contains two short introns.
Southern-type hybridizations (Figure 2) indicated three different DNA fragments with sequence homology to the cDNA of the isolated OsHKT1 in the rice Pokkali and IR29 genomes. Identical hybridization patterns were obtained from the genomes of both rice cultivars. Based on a genome size of 560 Mb for rice, a copy number reconstitution also indicated a signal strength equivalent to at least two copies. Also, restriction digests with several enzymes, the sites of which are not present in the sequenced cDNA, highlighted gene-size fragments whose intensity indicated at least two HKT1-homologous sequences in the rice genome. In fact, two different cDNAs with 91% identity to each other have been isolated from line Pokkali (Horie et al., 2001).
Rice HKT1 complements yeast mutants defective in K+ uptake
Yeast complementation was used to monitor the potential function of OsHKT1 in K+ transport (Figure 3). A mutant, CY162, which is deficient in TRK1 and TRK2 (Ko and Gaber, 1991) grows at 100 mm KCl. This strain, as well as a strain transformed with vector only, is unable to grow on 10 mm KCl. When the OsHKT1 coding region under control of the inducible Gal1-promoter was transformed into CY162, cells were unable to grow at low K+ in the absence of galactose (Figure 3a). Induction of the promoter by galactose (arginine/phosphate-based medium; Rodriguez-Navarro and Ramos, 1984) allowed growth in as low as 300 µm KCl (Figure 3a). Addition of 150 mm NaCl inhibited growth on plates and in liquid culture at 0.3 mm KCl (Figure 3b). CY162 transformed with the empty pYES2 vector did not grow in the presence of galactose on agar plates and in liquid culture when KCl in concentrations of 7 mm or less was present (not shown).
Electrophysiological characterization of OsHKT1
The electrophysiological characterization of OsHKT1 was performed following methods established for wheat HKT (Gassmann et al., 1996; Rubio et al., 1995). Figure 4a shows that perfusion of the solutions containing 1.0 mm Na+, 0.3 mm K+ plus 1.0 mm Na+ or 0.3 mm K+ over an oocyte injected with water and held at the resting potential of − 40 mV, did not cause any appreciable changes in the current levels. In contrast, perfusion of the same solutions over an oocyte injected with OsHKT1 and held at the resting potential of − 160 mV induced the activation of inward currents. These currents were larger in the presence of 1.0 mm Na+ plus 0.3 mm K+ than in the presence of 0.3 mm K+ or 1.0 mm Na+ alone. (Figure 4b). The expression of OsHKT1 caused polarization of the oocytes towards more negative potentials in the absence of Na+ and K+. This is probably due to the release of Na+ and/or K+ that has accumulated previously, as reported (Gassmann et al., 1996; Rubio et al., 1995), thus explaining the more negative holding potential for the injected oocyte (Figure 4b). We also characterized the HKT-induced currents by determining the reversal potential under two conditions, maintaining extracellular K+ constant at 1 mm, while varying Na+, and vice versa, keeping external Na+ at 1 mm while changing K+ (Rubio et al., 1995). With constant Na+ (1 mm), increases in K+ positively shifted the reversal potential by 21 mV when K+ was changed from 1 mm to 10 mm (Figure 5a). In contrast, when K+ was kept constant at 1 mm and Na+ varied between 1 mm and 10 mm the reversal potential shifted positively by 15 mV (Figure 5b). The wheat HKT1 co-transporter is highly selective towards K+ and Na+, allowing the movement of these two cations when present together, but it also mediates the movement of Na+ in the absence of K+ when Na+ is found at millimolar levels (Gassmann et al., 1996; Rubio et al., 1995). The Arabidopsis homologue, AtHKT1 was shown to function as a Na+ uptake transporter independent of external K+ when expressed in Xenopus oocytes (Uozumi et al., 2000). The AtHKT1-mediated current was not affected by the external K+ concentration (Uozumi et al., 2000). Figure 5(c) shows the current-voltage relationships of a rice HKT1-injected oocyte exposed to alkali cations at concentrations of 100 mm. This figure demonstrates that in the presence of any of these cations, hyperpolarization of the oocyte induced inward currents in all cases, with a permeability sequence of Rb+ > Cs+ > Na+ > K+ > Li+. In contrast, the currents induced by the alkali cations in water injected oocytes were below 50 nA with a permeability sequence Na+ > K+ > Li+ > Rb+ > Cs+ (data not shown). These results are in contrast to reports on wheat HKT1 (Gassmann et al., 1996; Rubio et al., 1995) where inward currents were recorded only in the presence of 100 mm Na+ (however, see Schachtman and Schroeder, 1994). These results are also different from those reported by Horie et al. (2001) for japonica rice, cv. Nipponbare (Ni-OsHKT1) who reported a permeability sequence of Na+ > Li+ > K+ ≥ Rb+ > Cs+.
HKT1 transcript abundance in different rice lines
Expression of OsHKT1 in IR29 and Pokkali depending on K+ nutrition and salt stress was analysed by RT-PCR amplification with gene-specific oligonucleotide primers that were designed for the coding region and the non-coding region of the gene, respectively (Figure 6). In plants adapted to nutrition medium without K+, added expression of OsHKT1 was detected in leaves and roots. Following treatment with 150 mm NaCl for 24 h, transcription of OsHKT1 was reduced in leaves and roots of both rice lines.
To study effects of increasing external K+ on the expression of OsHKT1, the plants were adapted to medium containing 4 mm K+. By RT-PCR, amplification signals of OsHKT1 were consistently detected in leaf (not shown) and root tissue of both rice lines (Figure 7). Two signals were obtained in transcripts from IR29 roots. Sequencing of the additional (slightly larger) cDNA-fragment revealed this band as a differently spliced OsHKT1 that included an intron of 291 bp inserted between bp 1388 and 1389 of the cDNA (not shown; accession: AF450299, AF313388). The presence of this intron is confirmed by the analysis of the rice genome sequence available of NCBI (HKT located on PAC clone #AP003726). The altered transcript includes two in-frame stop codons indicating that no functional OsHKT1 protein will be translated. By exposure to 150 mm NaCl for 24 h OsHKT1 signals were reduced in roots of both rice lines.
Cell-specific expression of rice HKT1
Cell-specificity of expression of OsHKT1 was studied by in situ hybridizations in the rice lines IR29 and Pokkali adapted to 0 mm K+. In root tips, IR29 showed signals in the epidermis and exodermis, less in the cortex and strong expression in the vascular cylinder (Figure 8). In the vascular cylinder, signals were of approximately similar strength in endodermis, pericycle, phloem and xylem parenchyma cells. Pokkali showed little expression of OsHKT1 in the vascular cylinder, but patchy expression in root epidermis, exodermis and adjacent cells of the cortex. In mature roots, expression in IR29 was observed in epidermis, exodermis and the developing sclerenchymatic cells, as well as in the endodermis and the cells inside. Pokkali showed hybridization to the exodermis and weak signals in selected cells neighbouring the xylem vessels.
For in situ hybridizations in leaves the region around the midrib of the second youngest leaf from 3-week-old plants was used. Signals were similar in IR29 and Pokkali, with strongest expression in regions between the phloem and xylem and the transition from vasculature to mesophyll cells.
In root tips of both varieties, when adapted to medium without K+ added, salt stress of 150 mm NaCl decreased the signal to background levels within 24 h (Figure 9). Salt stress did not abolish OsHKT1 signals in the mature root as it did in the root tip. In IR29, signals in the epidermis and in part also in the exodermis disappeared or became weaker, but strong signals were still observed with sclerenchymatic cells, which still contain cytoplasm. In the vasculature, the endodermis stained weakly, but signals persisted in xylem parenchyma cells. OsHKT1 expression in Pokkali was nearly exclusively restricted to the sclerenchymatic cells subtending the exodermis, and to xylem parenchyma cells in the centre of the vascular stele. In leaves, OsHKT1 signals increased in the phloem of IR29 plants under salt stress but not in Pokkali (not shown).
Effects of alkali cations on the expression of OsHKT1
Plants adapted to 0 mm K+ were treated with 150 mm each of RbCl, LiCl, and CsCl, for 24 h followed by RT-PCR amplification of OsHKT1. The OsHKT1 transcript level was reduced by treatment with all three cations in roots (Figure 10) and leaves (not shown) of Pokkali and by Li+-treatment alone in roots (Figure 10) and leaves (not shown) of IR29. Rb+ and Cs+ induced alternate splicing of OsHKT1 in roots and leaves of IR29 as it was similarly observed in roots of IR29 plants adapted to 4 mm K+ (compare with Figure 7). Comparison of ion contents showed slightly increased K+, Li+ and Rb+ amounts in leaves of IR29 treated with 150 mm LiCl and 150 mm RbCl, respectively, for 24 h. In Pokkali, the K+ content was not changed in comparison to control plants and the amounts of Li+ and Rb+ accumulated were lower than the concentrations in IR29 (Figure 11).
We have isolated and characterized a homologue of wheat HKT1 from the indica rice, IR36. OsHKT1 expression in the salt-sensitive rice line IR29 and the salt-tolerant line Pokkali was compared to begin to understand the possible role of this transporter in the homeostasis of monovalent alkali cations. OsHKT1 shares 65% identity with the wheat homologue, 39% identity with Arabidopsis AtHKT1, 42% with Eucalyptus EcHKT1, and 46.5% with ice plant McHKT1 (Su, 2001). Recently, sequences of OsHKT1 have been isolated from two rice varieties, the japonica rice, Nipponbare, and Pokkali (Horie et al., 2001). Both are identical to the IR36 HKT1 at the amino acid level, and only 4 nucleotide differences exist in the coding region between HKT1 of the lines Nipponbare, Pokkali and IR29. OsHKT1 appears to be encoded by a single gene in Nipponbare but a second transcript, OsHKT2, with 91% identity to OsHKT1, has been isolated from a Pokkali cDNA library (Horie et al., 2001). Our hybridizations indicated the existence of a small gene family of 2–3 HKT-type sequences in both Pokkali and IR29. Possibly, HKT1 copy number distinguishes indica and japonica rice.
Functional analyses of HKT-type transporters have been performed by heterologous expression in yeast and bacterial strains deficient in K+-uptake and in Xenopus oocytes. The wheat HKT1, for example, is highly selective for K+ and Na+ acting as a Na+-coupled K+ uptake transporter. At high millimolar concentrations of Na+, low-affinity Na+-uptake is mediated and high-affinity K+-uptake is blocked (Gassmann et al., 1996; Rubio et al., 1995). For Arabidopsis AtHKT1 expressed in Xenopus oocytes, large Na+ currents were observed but no significant uptake of K+ Li+, Rb+, or Cs+ occurred (Uozumi et al., 2000). Conversely, the Eucalyptus homologues EcHKT1 and EcHKT2 mediated alkali cation uptake with a higher permeability shown towards Na+ and K+ when heterologously expressed in Xenopus oocytes (Liu et al., 2001). Horie et al. (2001) reported OsHKT1 identified from rice Nipponbare and from indica Pokkali to be a Na+ transporter and OsHKT2 in Pokkali to be a Na+/K+-coupled transporter based on functional analyses in Xenopus oocytes and yeast. Interestingly, in the present study we found that the expression of OsHKT1 conferred not only Na+ hypersensitivity to a yeast mutant deficient in the TRK1/TRK2 K+ transport systems but also K+ uptake was restored enabling the mutant to grow on µm and mm K+ concentrations. In contrast to the experiments on yeast complementation with OsHKT1 (Horie et al., 2001) we used a medium lacking NH4+ but containing arginine as the nitrogen source because competition of NH4+ with K+ transport has been reported in yeast (Rodriguez-Navarro and Ramos, 1984). These results indicate that the OsHKT1-mediated K+-transport in yeast is NH4+-sensitive and may be inhibited by the ion. This is in contrast to the insensitivity to external NH4+ of wheat HKT1 when the transporter is expressed in yeast (Santa-Maria et al., 2000).
OsHKT1 transcripts are expressed in specific cells and respond to alkali cations
OsHKT1 was expressed in leaves and roots of IR29 and Pokkali, but in situ hybridizations indicated differences in cell-specificity. The expression of OsHKT1 in the rice root in the two lines is observed in different cell layers. In the tip region, some signal was localized to the epidermis, but strongest expression in both varieties was seen in a layer of cells subtending the epidermis. Cortex cells had fewer signals in contrast to what has been reported for wheat HKT1 (Schachtman and Schroeder, 1994). A consistent difference between the rice varieties was seen for hybridization to cells of the vascular tissue. In IR29, strong signals to all cells of the developing vascular cylinder, strongest to xylem parenchyma cells, contrasted with less OsHKT1-signal in the vasculature in Pokkali. In addition, Pokkali signals generally appeared in patches while IR29 stained more uniformly. A possible interpretation of this cell specificity would be general transport of K+ towards the vasculature in the tip but lateral movement of K+ in the mature root, consistent with our physiological data on ion uptake into the roots (see also, Schachtman and Schroeder, 1994). In leaves, OsHKT1 signals were found in cells adjacent to phloem vessels suggesting involvement of these transporters in K+-loading of the phloem. Based on kinetic studies in rye, White (1997) suggested that, depending on the K+ concentration in shoot and root phloem, the K+ uptake rate into roots is regulated.
After NaCl stress, OsHKT1 transcription was down-regulated in root-tips of both rice varieties as shown by in situ hybridizations. In mature roots the expression was reduced in the epidermis and vascular tissue with weaker transcript levels in Pokkali than in IR29. OsHKT RT-PCR fragments were then used to probe for expression differences under salinity stress distinguishing the two lines. For the coding region of the gene, inhibition of the expression of OsHKT in the roots of both rice lines by salt treatment was shown. Signals from the untranslated gene-specific region of OsHKT1 indicated that in IR29 the specific expression of OsHKT1 was not affected whereas in Pokkali the expression of the gene was inhibited. This comparison is valid because the 3′ untranslated regions of HKT are sequence-identical in IR29, Pokkali and Nipponbare in those segments that are found in all three genes (Horie et al., 2001).
OsHKT1 transcripts were detected in leaves and roots of IR29 and Pokkali after adaptation to both medium without K+ and medium containing millimolar concentrations of K+. Horie et al. (2001) reported OsHKT1 transcripts in the varieties Nipponbare and Pokkali to be higher in low potassium. In Arabidopsis, transcription levels of AtHKT1 were not affected by the addition of up to 100 mm K+ or 100 mm Na+ (Uozumi et al., 2000) whereas in roots of wheat and barley accumulation of HKT1 mRNA was stimulated upon K+ starvation (Wang et al., 1998). In barley this effect was most pronounced in young seedlings whereas at the age of 8- to 9-days HKT1 expression was only weakly stimulated by K+ starvation (Wang et al., 1998). It seems possible that reports on differences in the induction of HKT1 by K+ deprivation may have their basis in age- or development-specific regulation of these transporters (either amount, location in the membrane system or activity) and not in intrinsic qualities of the encoded proteins.
Effects of alkali cations on function and expression of OsHKT, respectively, were investigated by functional studies in Xenopus oocytes and by monitoring the transcript level of OsHKT1 in rice plants treated with different cations. In Xenopus oocytes expressing OsHKT1 inward currents were induced not only by K+ but all alkali cations tested with a permeability sequence Rb+ > Cs+ > Na+ > K+ > Li+, suggesting that OsHKT1 functions as a general alkali cation transporter. Permeability of HKT1 to all the alkali cations has been reported for wheat (Schachtman and Schroeder, 1994) and Eucalyptus camaldulensis (HKT1-2; Liu et al., 2001), although with a different permeability sequence. We have obtained similar results with the Mesembryanthemum crystallinum and Triticum aestivum HKT1 homologues (unpublished results). The permeablity sequence we report here for rice HKT1 contrasts with that of Horie et al. (2001) who, using the same clone, showed a greater permeability to Na+ compared with the other alkali cations. This difference is difficult to reconcile, however, our results were consistently observed in more than five oocytes from each of three different frogs with low se (Figure 5). Interestingly, Horie et al. (2001) proposed that the differences in selectivity between OsHKT1 (a Na+ transporter) and OsHKT2 (a Na+/K+ co-transporter) were due to a single amino acid change (Gly-88 in OsHKT2, Ser-88 in OsHKT1). However, this observation is not supported by our results with rice HKT and those of Feirbain et al. (2000) and Liu et al. (2001) for the two HKT orthologues from E. camaldulensis, which function as Na+/K+ co-transporters where the equivalent amino acid corresponds to Ser-95 (EcHKT1) and Ser-94 (EcHKT2). It may be argued that the currents reported here are smaller than those reported by Rubio et al. (1995), Gassman et al. (1996) and, in part, Horie et al. (2001). However, as a control we have injected the mammalian brain K+ channel Kv1.1 into Xenopus oocytes and have recorded currents larger than 10 µA at 30 mV (data not shown), similar to those reported by other researchers for the same channel (Rolf et al., 2000). Also, current levels similar to those reported by us for the rice HKT homologue were recorded by Schachtman and Schroeder (1994) for wheat, Horie et al. (2001) for rice, and those by Feirbain et al. (2000) and Liu et al. (2001) for the two HKT1 orthologues from E. camaldulensis. In root and leaf tissue of both rice lines OsHKT1 expression was inhibited by exposure of plants to Na+ and Li+. In Pokkali, Rb+ and Cs+ induced down-regulation of the transcripts. In IR29, in the presence of millimolar concentrations of K+ (representing excess), and in the presence of Cs+ and Rb+, we observed a different RT-PCR fragment based on the presence of an intron. This suggests incomplete splicing as a possible mechanism by which transcripts might be retained in the nucleus or by which translatable transcripts are reduced under stress conditions. OsHKT1 clearly acts as a general alkali cation transporter not only when expressed in Xenopus oocytes but also after expression in planta.
Salt-dependent regulation of OsHKT1 expression in rice
Molecular mechanisms governing Na+ uptake into plants are not well understood, but experimental data suggest non-specific influx through K+ uptake systems (Amtmann and Sanders, 1999; Blumwald et al., 2000; Lagarde et al., 1996; Maathuis and Sanders, 1997; Maathuis et al., 1997; Schroeder et al., 1994). Several protein families could account for inadvertent Na+ uptake-low-affinity channels, dual- or high-affinity transporters and voltage-independent cation channels (VIC). Another possible uptake vehicle are non-selective cation channels (Schachtman and Liu, 1999; White, 1997) and proteins like the wheat low-affinity cation transporter LCT1 (Schachtman et al., 1997). Many low-affinity channels (AKT/KAT-type) are highly selective for K+, but other monovalent cations are not completely excluded (Blumwald et al., 2000; Maathuis et al., 1997; Schachtman et al., 1992). The expression of the homologue of inward-rectifying K+ channels OsAKT1 was not modified in the root epidermis and endodermis of IR29 under salt stress conditions but was inhibited in Pokkali (D Golldack, F Quigley, CB Michalowski, U Kamasani and HJ Bohnert; unpublished). Dual-affinity K+-transporters (HAK/AT/KUP-type) have recently been shown to be affected by Na+, or they may act as low-affinity Na+ transporters (Fu and Luan, 1998; Quintero and Blatt, 1997; Santa-Maria et al., 1997; Su, 2001). The K+ uptake protein represented by HKT1 from wheat has been shown to function as a K+/Na+-co-transporter. Its action as a low-affinity uptake protein in the presence of 100 mm Na+ (Rubio et al., 1995) could explain Na+ accumulation. Our results with rice showed a difference between two rice lines with respect to the expression of OsHKT in the root vascular tissue whereas expression in the root tip was identical. The difference correlated with salt sensitivity in IR29 and tolerance in Pokkali, indicating a possible involvement of OsHKT1 in uptake and long-distance transport of cations in IR29 that is probably realised in Pokkali to a much lower degree. As a response to alkali-salt stress OsHKT1 transcription was down-regulated in roots and leaves of Pokkali, whereas that in IR29 was either unchanged or down-regulated to a minor extent (Figures 6 and 8). If OsHKT1 functions as a Na+ transporter (Figures 4 and 5), our in situ and RT-PCR results (Figures 6,8,9 and 10) suggest that regulation of OsHKT1 by salt stress may be an important mechanism in salt tolerance. This view is supported by the consistent down-regulation of OsHKT1 caused by salt stress in the salt tolerant rice line Pokkali, leading to a lower level of Na+ (Li+ and Rb+) accumulation in planta, as compared to the amount taken up by the salt sensitive rice line IR29 (Figures 1 and 11). Further support comes from the recent detection of an Arabidopsis mutant in which salinity stress tolerance could be based on down-regulation (knockout) of the AtHKT1 gene and protein (Rus et al., 2001). Our study suggests different cell-specificity for HKT transcripts and, also, intraspecies specific differences under high salt conditions that distinguish the Na+-tolerant line Pokkali from Na+-sensitive IR29. Accordingly, HKT-type transporters may contribute to the differences in Na+-uptake behaviour of these rice lines in high salinity. Possibly of high significance is that OsHKT seems to be involved in xylem loading and long distance transport of alkali cations from roots to shoots and in leaf phloem loading. The OsHKT transporter may thus determine how much Na+ is ultimately taken up and transported to leaves in a feedback regulation influencing other K+/Na+ uptake systems in high salinity. The differences in OsHKT1 expression between IR29 and Pokkali observed under salt stress conditions may be due to variations in promoter sequence, transcription factors, or in, as yet unknown, signalling events that control expression. A comparative analysis of such a control system in Pokkali and IR29 will be necessary to gauge the relative importance of HKT1 in K+ homeostasis or Na+ exclusion and uptake in rice.
Seeds of rice (Oryza sativa L. indica) var. IR29 and Pokkali were obtained from IRRI (Los Banos, Philippines), germinated in sand, transferred to aerated hydroponic tanks and grown for 3 weeks in all experiments (12-h light/dark at 28°C/22°C) in controlled environment chambers (Conviron, Winnipeg, Canada). Plants were grown in half-strength Hoagland's nutrient solution with K+ concentrations as indicated for the experiments. The addition of chelated iron was doubled. In experiments with reduced K+ the NO3– counter ion was substituted by NH4+. For salt stress experiments, the nutrient solution contained 150 mm NaCl (or LiCl, RbCl or CsCl) for 24 h and 72 h, respectively. Unstressed control plants were grown in parallel and harvested at the same time.
Cloning and sequencing
A partial cDNA clone with homology to wheat HKT1 was obtained from the Cornell Collection (clone RZ405) and sequenced with an automated ABI sequencer (model 377, version 2.1.1). Amplification of the 5′-end was done by 5′RACE-PCR (Gibco-BRL, Life Technologies, Inc., Rockville, MD, USA). The full-length cDNA was assembled and cloned in the XhoI and HindIII restriction sites of pBluescript (Stratagene, La Jolla, CA, USA) and the entire cDNA-fragment was re-sequenced. The sequence of OsHKT1 was deposited at GenBank (AF313388).
Genomic DNA was extracted from IR29 and Pokkali leaves according to Gustinich et al. (1991). DNA blotting and hybridization were performed according to standard procedures (Sambrook et al., 1989). The probe, corresponding to nucleotides #283-1779 of the full-length clone, was labelled with [32P]dCTP (Amersham-Pharmacia Biotech Inc., Piscataway, NJ, USA) by random priming. After hybridization in 2× SSC, filters were washed with 0.2× SSC at 42°C for 30 min. For an estimation of the copy number in the rice genome (560 Mb) plasmid DNA was loaded on the gels to produce signals equivalent to 0.5–4 copies.
In situ hybridizations
Root and leaf tissues were fixed with FAA, dehydrated and embedded. Root tips and segments approximately 5 cm from the tip were taken. Leaf sections were probed from the mid-section of the second youngest leaf of each plant. For tissue embedding Paraplast Plus (Fisher Scientific, Pittsburgh, PA, USA) was used. Sections 10 µm thick were mounted on poly-l-lysine coated slides. Sense and antisense RNA transcripts were synthesized by T3 and T7 RNA-polymerase from linearized pBluescript harbouring the cDNA (nucleotide #283-1779) using digoxigenin-UTP (Boehringer, Mannheim, Germany) as a label. Transcripts were hydrolyzed by alkaline treatment to an average length of 200 nucleotides. In situ hybridizations were done according to Golldack and Dietz (2001) and signals detected by antidigoxigenin alkaline phosphatase-conjugated Fab fragments (Boehringer, Mannheim, Germany) and 5-bromo-4-chloro-3-indolyl-phosphate and Nitroblue tetrazolium as a substrate. Microscopic analyses were carried out using an Axiophot fluorescence microscope (Zeiss, Oberkochen Germany). Photographs were processed through Nikon Scan 2.1 and Adobe Photoshop.
RNA from root and leaf tissue of rice (IR29 and Pokkali) was isolated by guanidinium thioisocyanate extraction according to Chomczynski and Sacci (1987). cDNA was synthesized from total RNA using Superscript RT II (Gibco-BRL). Equal aliquots of cDNA were used as template for amplification in 50 µl standard PCR reactions. A sequence specific forward primer (5′-GCACTG TCCAATGGAGAT-3′; position 1192-1209) and a reverse primer (5′-TTCCTTCATCACTCCACC-3′, position 1493-1510) were used for the amplification of a fragment of the 3′ translated region of the sequence. A partial fragment of the 3′ non-translated region was amplified using the same forward primer and the reverse primer 5′-TGCACTTGATGCACTTGC-3′ (position 1626-1643). The PCR amplifications were carried out as described before (Golldack and Dietz, 2001). A PCR-fragment with increased size from IR29 plants was cloned into PCR2.1 TOPO (Invitrogen, Carlsbad, CA, USA) and sequenced.
Analysis of ion contents
Root tissue and the second leaf of each plant were collected and ground in liquid nitrogen. After homogenization in ethanol : chloroform : water (12 : 5 : 3) and re-extraction with water, the aqueous phase was used for cation HPLC analysis (IonPac cation exchange; Dionex, Sunnyvale, CA, USA) (Adams et al., 1992). Alternatively, analyses of ion concentrations were performed with an inductively coupled plasma atomic emission spectrometer (ICP-AES, Perkin-Elmer, Boston, MA, USA) according to Brune et al. (1995). The results shown are means from 5 plants.
The full-length cDNA of OsHKT1 was cloned in the BamHI and XhoI restriction sites of pYES2 (Invitrogen) under control of the inducible Gal1-promoter into S. cerevisiae CY162 (Gietz et al., 1992; Ko and Garber, 1991). Initial selections of transformants were done on glucose containing SC-agar plates without uracil supplemented with 100 mm K+. Positive transformants were replica-plated on galactose containing arginine/phosphate medium (Rodriguez-Navarro and Ramos, 1984) without uracil containing 7 mm K+ and selected a second time on arginine-phosphate top-agar plates without uracil supplemented with galactose and 300 µm K+. For growth studies, positive transformants were transferred to arginine/phosphate-based liquid medium of K+ concentrations as indicated. Growth experiments, repeated at least three times with different transformants, were carried out in 20 ml aliquots and samples of 0.3 ml were taken to measure OD600 (nm). The results shown are taken from one representative experiment. The plasmid was re-extracted from positive transformants and maintained in E. coli (d'Enfert et al., 1995). As a control, S. cerevisiae CY162 was transformed with the pYES2 vector without any insert.
Preparation of template DNA, in vitro transcription and capping of mRNA
The coding region of the OsHKT1 gene was cloned into the pGEM-HE vector which contains a T7 RNA polymerase promoter and 5′- and 3′-UTR of the Xenopus β-globin gene for enhanced expression (Liman et al., 1992). The plasmid DNA was purified by CsCl gradient ultracentrifugation and linearized with NheI that cleaves the plasmid downstream of the inserted cDNA. OsHKT1 RNA was synthesized in vitro using the mCAP(TM) mRNA Capping Kit (Stratagene) a procedure, which tends to enhance the translation efficiency of RNA transcripts in Xenopus oocytes.
Two-electrode voltage clamping of Xenopus oocytes
Oocytes injected with 25 ng of mRNA were used 2–5 days after injection for the recording of currents induced by the expression of HKT1 with the two-electrode voltage clamp technique. Initially, the voltage electrode was introduced into the oocyte. After a stable membrane potential had been reached for several minutes, the current electrode was inserted. The electrodes were filled with 1 m KC1 and the oocytes bathed with a solution containing 6 mm MgC12, 1.8 mm CaC12, 10 mm Tris/MES, pH 5.5, with osmolality adjusted to 240–260 mOsmol kg−1 with d-mannitol. K+ and Na+ were added as glutamate salts in experiments where only these two cations were studied. In the experiments where the selectivity of HKT1 was assayed, different alkali cations were added as chloride solutions. Recordings were obtained with a Dagan Oocyte Clamp amplifier (Dagan, Minneapolis, MI, USA); data acquisition and analysis were done with program pCLAMP6 (Axon Instruments, Foster City, CA, USA).
We are grateful to Ms. W. Chmara (University of Arizona, AZ, USA) for HPLC analysis, to Ms. C. B. Michalowski (University of Arizona) for help with sequencing, to Ms. E. Reisberg (Universität Würzburg) for ICP-AES measurements. We also wish to thank Dr B. Barkla (UNAM) for help with the manuscript, and Dr A. Darszon (UNAM) for providing a two-voltage clamp amplifier. We are indebted to Dr R. Gaber (University of Chicago) for providing yeast strains. The project has been supported in part by the Arizona Agricultural Experiment Station. D.G. acknowledges support by the Deutsche Forschungsgemeinschaft, Bonn, Germany. U.R.K. has been supported by a grant from the Rockefeller Foundation, New York, USA. CONACyT, Mexico, and the National Science Foundation (International Programs), USA, supported collaborative work (to O.P and H.J.B), CONACyT, Mexico, and the BMBF (International Bureau), Germany, supported collaborative work (to O.P and D.G). O.P. received additional support from CONACyT (Grant 33054-N).